In the semiconductor industry, there is a continuing trend toward higher device densities. Fabrication of very large scale integrated circuits (VLSI) and ultra large scale integrated circuits (ULSI) requires that resist materials, lithographic processes, and exposure tools meet necessary performance demands for high throughput manufacturing of sub-micron feature size devices. In particular, the semiconductor industry is producing, with increasing frequency, integrated circuits having structures which are markedly less than 1 μm. The increased integration density increases the requirements imposed on the photolithographic process.
Resist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures. For exposure apparatuses using short-wavelength light, such as an excimer laser, chemical amplification type resists are often employed. The chemical amplification type resists generally consist of a resin, a photosensitive acid generator, and a solubilizer or a cross-linking agent. The acid generator generates an acid upon exposure. During post-exposure baking (PEB), this acid functions as a catalyst to encourage the reaction of the solubilizer or the cross-linking agent, and a pattern is formed by development. A photoresist using the solubilizer forms positive patterns, and a photoresist using the cross-linking agent forms negative patterns.
One concern regarding utilization of chemically amplified resists is that their profile is degraded by airborne and substrate contaminants such as amines, ammonia, n-methylpyrrolidone, and tetramethylammonium hydroxide. Airborne contaminants produce T-top effects and alkaline substrate contaminants cause footing effects.
Footing occurs when a resist that remains after development does not have substantially vertical walls. Instead, a foot is formed at the bottom of the wall of the resist when the developer does not remove all of the resist from a desired opening. Consequently, the opening gets smaller (small cross-section) as the depth in the resist increases. FIG. 1 illustrates a patterned semiconductor device 100 on a substrate 102 exhibiting footing. The opening through the resist has a foot 108 at the interface between the resist layer 106 and the antireflective coating layer 104.
During post-exposure bake, the catalytic action of an acid generated by irradiation is susceptible to an undesirable reaction with basic compounds. Especially in the case of positive photoresist, if a basic gas such as ammonia gas or amine gas is present in the atmosphere between an exposure apparatus and a post-exposure bake apparatus, an acid generated by irradiation reacts with this basic gas and is neutralized. In addition, photo generated acid near the surface layer is susceptible to evaporation. Neutralization and/or evaporation of acids alter the pattern formation in the resist resulting in the formation of T-tops (a T-shaped overhang in the upper portion of the resist). FIG. 2 illustrates a patterned semiconductor device 200 exhibiting T-tops formation. The opening through the resist has a T-tops 208 at the upper portion of the resist. The T-tops 208 cause the opening near the surface of the resist 106 to be narrower at the top portion of the resist.
Despite many attempts to understand and prevent footing and for T-tops in resist processing, there remains a need for more effective and efficient methods of eliminating these defects.